Method, system and computer program for comparing images

ABSTRACT

A method of determining whether a user of a mobile device corresponds to a previously authenticated user. The method comprises: acquiring a first image from the identity document, the first image being a photographic image and or a digital image; causing a camera connected to or integral with the mobile device to capture a second image, the second image corresponding to a user of the mobile device; comparing data indicative of the first image and data indicative of the second image, whereby to determine whether the first image and the second image represent the same user; and responsive to determining that the first image and the second image represent the same user, forming an association between the previously authenticated user and the mobile device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/595,559, filed May 15, 2017, which is a continuation of U.S.application Ser. No. 14/751,051, filed Jun. 25, 2015, which is acontinuation under 35 U.S.C. § 120 of International Application No.PCT/GB2014/050993, filed Mar. 28, 2014 (published by the InternationalBureau as International Publication No. WO/2014/155130 on Oct. 2, 2014),which claims priority to (1) GB Application No. 1305814.4, filed Mar.28, 2013; (2) U.S. patent application Ser. No. 13/953,619, filed Jul.29, 2013; and (3) GB Application No. 1319344.6, filed Nov. 1, 2013. Theentire contents of the above-referenced patent applications are herebyincorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method, system and computer programfor comparing images.

Description of the Related Technology

There is growing demand for services providers to provide their servicesvia devices, such as PCs, tablets and mobile phones. However, for manyservice providers, the need to verify the credentials of the users towhom they are providing a service is very important. For providers ofcertain online services, for example, there is a need to ensure thattheir users are above a certain age. As an example, online bankingservice providers need to ensure that the identity of a user is reliablyverified before that user is allowed access to user-restricted bankingservices. There are particular challenges when verifying the identity ofa user via a device, compared to, for example, verifying a person'sidentity in via a face-to-face transaction.

SUMMARY

According to a first aspect of the present invention, there is provideda method of determining whether a user of a mobile device corresponds toa previously authenticated user, the user having been previouslyauthenticated via an identity document comprising: a photographic imageof the previously authenticated user, the photographic image beingvisible on said identity document; and an integrated circuit componentstoring data representative of a digital image of the previouslyauthenticated user, the method comprising: causing a chip readerconnected to or integral with the mobile device to access the integratedcircuit component, whereby to retrieve said data representative of adigital image of the previously authenticated user; causing a cameraconnected to or integral with the mobile device to capture a firstimage, the first image corresponding to an image of a portion of theidentity document containing said photographic image visible on theidentity document; causing a camera connected to or integral with themobile device to capture a second image, the second image correspondingto a user of the mobile device; and, arranging for said retrieved dataand data indicative of said first and second images to be compared,whereby to determine whether the first image, the second image, and thedigital image represent the same user; and, in the event that it isdetermined that the first image, the second image, and the digital imagerepresent the same user, forming an association between the previouslyauthenticated user, and the mobile device.

The step of comparing the retrieved data and the data indicative of thefirst and second images, may be performed on the basis of every possiblepermutation of the retrieved data and the data indicative of the firstand second images. Alternatively, a less processor intensive process maybe employed, in which the retrieved data is compared to the dataindicative of the first image, and separately to the data indicative ofthe second image.

By forming an association between the previously authenticated user andthe mobile device, the mobile device is, in effect, verified as themobile device of the previously authenticated user. The association maybe used for several purposes.

As a first example, the association may be used by a third party whoprovides access to user-restricted services or resources via mobiledevices. More specifically, the association may be used by a third partyto determine the mobile device on which to provide access to aservice/resource that has been requested by the previously authenticateduser. In this case, upon determining that the first image, the secondimage, and the digital image represent the same user, the third partymay be informed of the mobile device with which the previouslyauthenticated user is associated. In this way, the third party can besure that the services or resources they provide are being supplied to amobile device that is held by the previously authenticated user.

As another example, the association may be used to identify suspicioususer behavior. For example, if a user of a first mobile device ispurporting to be a previously authenticated user; however, thepreviously authenticated user is associated with a different mobiledevice, and has only ever authenticated himself on that device, then theuser of the first mobile device may be identified as a suspicious user.In this case, more stringent verification checks may be carried out onthe identity document supplied by the user of the first device.

As a further example, the association may be used to authenticate theuser of the mobile device in a subsequent authentication event for thatdevice. More specifically, upon determining that the first image, thesecond image, and the digital image represent the same user, the datarepresentative of the second image, and/or the data representative ofthe digital image retrieved from the chip of the identity document, maybe stored as a validated image of the previously authenticated user,together with an association with the mobile device.

In a subsequent authentication event for the mobile device, the storedimage of the previously authenticated user that is associated with themobile device may be retrieved and compared to a newly captured image ofthe user of the mobile device. In this way, it can be determined whetherthe current user of the mobile device is the previously authenticateduser associated with the mobile device without the user being requiredto supply an identity document. In this case, therefore, the associationbetween the mobile device and the previously authenticated user is, ineffect, an association between the mobile device and an image that hasbeen verified as an image of the previously authenticated user.

In the specific arrangement where the second image (i.e. the image ofthe previously authenticated user captured by the mobile device) isstored as a validated image of the previously authenticated user at astorage device remote from the mobile device, such an associationbetween the mobile device and the second is of particular use. This isbecause a particular user will typically have a plurality of mobiledevices on which he authenticates himself. Thus, the remote storagedevice may store multiple “second” images of the previouslyauthenticated user; each of which were captured by a different mobiledevice. By storing an association between each of the second images andthe mobile device that captured the second image, the “correct” secondimage can be retrieved from the storage device when the user attempts toauthenticate himself via one of those devices. In other words, when auser attempts to authenticate himself by a particular mobile device, thesecond image that was captured by that mobile device may be retrievedfrom the storage device and compared to a newly captured image of theuser of that mobile device. By comparing images that were captured bythe same device, the reliability of the image matching result can beimproved.

Irrespective of how the association is used, the association formedbetween the previously authenticated user, and the mobile device may beformed, for example, by storing an association between a unique deviceidentifier for the mobile device and data that uniquely identifies thepreviously authenticated user. The data that uniquely identifies thepreviously authenticated user could comprise data representative of adigital image of the previously authenticated user, as described above.The unique device identifier and the data that uniquely identifies thepreviously authenticated user may be stored by a storage device remotefrom the mobile device.

By determining whether the first image, the second image, and thedigital image represent the same user, it can be determined up to a highlevel of confidence whether the user of the mobile device is thepreviously authenticated user.

More specifically, by performing a three-way comparison between theretrieved data and the data indicative of the first and second images,the reliability of the image matching result is improved as compared toperforming a two-way comparison between e.g. the retrieved data and thesecond image.

Additionally, by comparing the data representative of the first image(i.e. the image of the photographic image of the previouslyauthenticated user visible on the identity document), and the datarepresentative of a digital image of the previously authenticated userstored on the integrated circuit component, the validity of the identitydocument can be verified. For example, any alterations made to thephotographic image visible on the identity document can be detected.Additional validity checks may also be performed.

Advantageously, the method may comprise arranging for the retrieved dataand the data indicative of the first image to be compared, for thepurposes of verifying the validity of the identity document. For examplethe identity document may further comprise first data, in which case themethod may further comprise arranging for the first data to be derivedfrom the identity document such that verification of the validity of theidentity document is performed on the basis of the first data.

In one example, at least some of said first data may be stored in theintegrated circuit component, and the step of arranging for the firstdata to be derived from the identity document may comprise causing achip reader connected to or integral with the mobile device to accessthe integrated circuit component to retrieve said first data. As aparticular example, the first data stored in the integrated circuitcomponent may comprise data that is signed by the authority that issuedthe identity document. In this case, the step of verifying the validityof the identity document on the basis of the first data may compriseverifying that the data has been signed by the issuing authority.

Additionally or alternatively, at least some of said first data may bevisible on said identity document, and said first image comprises aportion of the identity document containing said first data. In thiscase the step of arranging for the first data to be derived from theidentity document comprises either: analysing features within said firstimage, or sending said first image to a remote processing system that isconfigured to analyse features within said first image, whereby toderive said first data from the identity document.

As a particular example, some of the first data may be stored in theintegrated circuit component and some may be visible on the identitydocument. The data stored in the integrated circuit component may beencrypted with a key that is derivable from the data visible on theidentity document. In this case, the step of verifying the validity ofthe identity document may comprise deriving the visible data from theidentity document, and using the visible data to derive the key fordecrypting the data stored in the integrated circuit component. In thisway it can be verified, for example, that the visible data and/or thedata on the integrated circuit component has not been tampered with.

As a further example, at least some of the first data may comprise aunique identifier for the user associated with the identity document. Insuch arrangements the unique identifier may be used to retrieve furtherdata relating to the user from a storage device remote from the identitydocument. The retrieved data may be used to check the validity of theidentity document. As a specific example, the retrieved data maycomprise an image of the user associated with the identity document, andthe validity of the identity document can be checked by comparing theretrieved image to the image of the identity document (i.e. the “firstimage”), and/or the data representative of the digital image stored onthe chip, to verify that the identity document has not been tamperedwith. Additionally, or alternatively, the retrieved image can becompared to the image of the user of the device (i.e. the “secondimage”). This may be done to improve the reliability of the userauthentication result.

In one arrangement, the identity document may further comprise seconddata relating to the previously authenticated user, and the method mayfurther comprise arranging for said second data to be derived from theidentity document. The second data may be data that identifies thepreviously authenticated user, such as the name, date of birth, and/oraddress of the user. In the event that the first image, the secondimage, and the digital image are determined to represent the same user,this second data can be stored, together with the identifier for thepreviously authenticated user, for use in a subsequent authenticationevent for the user. Thus, the user need not provide this data in asubsequent authentication event.

In one example, at least some of said second data is stored in theintegrated circuit component, and said step of arranging for said seconddata to be derived from the identity document comprises causing a chipreader connected to or integral with the mobile device to access theintegrated circuit component, whereby to retrieve said second data.

Additionally, or alternatively, at least some of said second data may bevisible on said identity document, and said first image may comprise aportion of the identity document containing said second data. In thiscase, the step of arranging for said second data to be derived from theidentity document may comprise arranging for said second data to beextracted from the first image using Optical Character Recognition.

The second data may be a subset of the above-mentioned first data.

In one arrangement, the chip reader may utilise a Near FieldCommunication protocol to access the integrated circuit component.

The above-mentioned step of arranging for the comparison of the datarepresentative of a digital image of the previously authenticated userand the data indicative of the first and second images may, in somearrangements, comprise sending the data representative of a digitalimage of the previously authenticated user and the data indicative ofsaid first and second images to a remote processing system configured tocarry out said comparison. Alternatively, the data representative of adigital image of the previously authenticated user and the dataindicative of said first and second images may be compared by aprocessing system of the mobile device.

According to a second aspect of the present invention, there is provideda processing system for use in determining whether a user of a mobiledevice corresponds to a previously authenticated user, the user havingbeen previously authenticated via an identity document comprising: aphotographic image of the previously authenticated user, thephotographic image being visible on said identity document; and anintegrated circuit component storing data representative of a digitalimage of the previously authenticated user, the processing system beingconfigured to: cause a chip reader connected to or integral with themobile device to access the integrated circuit component, whereby toretrieve said data representative of a digital image of the previouslyauthenticated user; cause a camera connected to or integral with themobile device to capture a first image, the first image corresponding toan image of a portion of the identity document containing saidphotographic image visible on the identity document; cause a cameraconnected to or integral with the mobile device to capture a secondimage, the second image corresponding to a user of the mobile device;arrange for said retrieved data and data indicative of said first andsecond images to be compared, whereby to determine whether the firstimage, the second image, and the digital image represent the same user;and, in the event that it is determined that the first image, the secondimage, and the digital image represent the same user, form anassociation between the previously authenticated user, and the mobiledevice.

According to a third aspect of the present invention, there is provideda computer program for use in determining whether a user of a mobiledevice corresponds to a previously authenticated user, the user havingbeen previously authenticated via an identity document comprising: aphotographic image of the previously authenticated user, thephotographic image being visible on said identity document; and anintegrated circuit component storing data representative of a digitalimage of the previously authenticated user, and the computer programcomprising instructions such that when the computer program is executedon a processing system the processing system is configured to carry outa method according to the first aspect.

According to a fourth aspect of the present invention, there is provideda method of verifying, by a processing system, whether a user of adevice corresponds to a previously authenticated user, the processingsystem having access to a first image and a second image, the firstimage being an image of an identity document comprising an image of thepreviously authenticated user and data identifying the previouslyauthenticated user, the identity document having been validated inrespect of the previously authenticated user by a trusted authority,whereby to previously authenticate the user, and the processing systembeing arranged to derive, from the identity document, said dataidentifying the previously authenticated user, wherein the second imageis an image captured by the device, the method comprising: comparingsaid first image to said second image, whereby to determine whether theyare images of the same user; and, in the event that it is determinedthat the first and second images are images of the same user:designating one of the first and second images as the higher qualityimage; storing said designated image as a validated image of thepreviously authenticated user in a storage device together with anidentifier for the previously authenticated user, said designated imagebeing designated for use in a subsequent verification event for thepreviously authenticated user and; arranging for said derived data to bestored, together with said identifier for the previously authenticateduser, whereby to enable said data to be retrieved in the subsequentverification event for the previously authenticated user.

By storing the designated image in a storage device together with anassociation with the previously authenticated user, the higher-qualitydesignated image can be used as an alternative to the lower-qualityimage to verify whether a user of a device corresponds to the previouslyauthenticated user in a subsequent verification event.

Identity documents typically include pictures of the user with whichthey are associated; however, these pictures are typically of a verypoor quality for the purpose of facial image matching. Currentlyidentity documents are typically issued in the form of a card or otherreprographic medium; however, the present invention is equallyapplicable where the identity document has an electronic identitycomponent, for example. An example of such an electronic identitycomponent could be a chip within an identity document, which stores datasuch as a digital representation of an image of the user. As anotherexample, the electronic identity component could be a storage devicethat is remote from the identity document, and which stores data such asa digital image of the user.

Typically the second image, which is an image captured by the device,will be of a higher quality for the purpose of facial image matching.Thus, by storing the second image, and using the second image inpreference to an image of an identity document in a subsequentverification event, the reliability of the subsequent verificationresult can be improved.

As mentioned, the identity document also comprises data that identifiesthe user with which it is associated. The data could include, forexample, information, such as the name, date of birth, and/or address ofthe user. By arranging for this data to be stored, together with theidentifier for the previously authenticated user, this data can be laterretrieved in a subsequent authentication event for the user. Thus, theuser need not provide this data in a subsequent authentication event.

The data may, in some cases, be printed or otherwise presented on asurface of the identity document. In such an arrangement, the data canbe derived from the image of the identity document using opticalcharacter recognition, for example.

Additionally, or alternatively, an identity document may comprise a chipwhich stores data that identifies the previously authenticated user. Inthis case, the data may be derived from the identity document using, forexample, near field communication. More specifically, the device whichwas used to capture the image of the identity document may comprise anear field communication reader configured to retrieve the data storedin the chip of the identity document when in close proximity to thechip. In effect, therefore, in this arrangement, the processing systemderives the data stored in the chip indirectly, through the near fieldcommunication reader of the device.

The data stored in the chip within the identity document may comprise adigital image of the previously authenticated user. Where this is thecase, the digital image may be retrieved and compared with the firstimage to verify that the first image is an image of a valid identitydocument. By this method, the processing system is able to determinewhether the identity document, and in particular the picture of thepreviously authenticated user on the identity document, has beentampered with. The digital image from the chip may also be storedtogether with other data derived from the identity document and theidentifier for the previously authenticated user.

The data that is derived from the identity document may additionally oralternatively comprise a unique identifier for the user associated withthe identity document. In such arrangements the unique identifier may beused to retrieve further data relating to the user from a storage deviceremote from the identity document. The processing system may use theretrieved data to check the validity of the identity document. As aspecific example, the retrieved data may comprise an image of the userassociated with the identity document, and the validity of the identitydocument can be checked by comparing the retrieved image to the image ofthe identity document (i.e. the “first image”) to verify that thepicture on the identity document has not been tampered with.Additionally, or alternatively, the retrieved image can be compared tothe image of the user of the device (i.e. the “second image”). This maybe done to improve the reliability of the user authentication result.

In addition, or as an alternative, to using the retrieved data to checkthe validity of the identity document, the processing system may arrangefor the retrieved data to be stored such that it can be used insubsequent authentication events.

Advantageously, in a subsequent verification event for a user of thedevice, the method comprises comparing an image that has subsequentlybeen captured by said device as part of the subsequent verificationevent to said designated image whereby to determine whether the user ofthe device is said previously authenticated user. Using thehigher-quality designated image for the comparison improves thereliability of the result of the subsequent verification event.

In the event that it is determined, in the subsequent verificationevent, that the user of the device is the previously authenticated user,the method may, in one arrangement, comprise: using said identifier forthe previously authenticated user to retrieve the details derived fromthe first image; and, sending said details to a system remote from theprocessing system together with said identifier.

In one arrangement, the method comprises encoding said second imageusing a one-way encoding algorithm prior to storing the encoded secondimage at said step of storing the second image.

The previously authenticated user may be associated with a unique useridentifier, and in the event that it is determined that the first andsecond images represent the same user, the method may further comprisestoring an association between said unique user identifier and thesecond image. The unique user identifier may be used to retrieve thesecond image from the storage device if the previously authenticateduser subsequently wishes to authenticate himself in a subsequentverification event.

Furthermore, in the event that it is determined that the first andsecond images are images of the same user, the method may furthercomprise sending said identifier to a system remote from said processingsystem whereby to indicate that the user of the device has been verifiedas the user associated with said identifier. The system remote from theprocessing system may be, for example, a system associated with aservice provider with which the user of the device wishes toauthenticate himself.

In yet another arrangement, the device is associated with a uniquedevice identifier, and in the event that it is determined that the firstand second images are images of the same user, the method furthercomprises storing an association between said unique device identifierand the second image.

In the event that it is determined that the first and second images areimages of the same user, the method may further comprise sending saidunique device identifier to a system remote from said processing systemtogether with an indication that the user of the device has beenverified. This may be particularly useful where the system remote fromthe processing system is associated with a service provider as discussedabove, and the service provider provides services directly to devices.

In one arrangement, said step of designating one of the first and secondimages as the higher quality image comprises the step of comparing theimage quality of the first image to the image quality of the secondimage. Alternatively, it may be assumed that the second image is thehigher quality image by default, without performing a comparison.However, performing such a comparison of the image qualities of thefirst and second images ensures that the higher quality image can alwaysbe used in a subsequent verification event.

According to a fifth aspect of the present invention, there is provideda processing system for use in comparing two images in order todetermine whether they represent the same entity, the processing systembeing configured to: assess an image quality of each of a plurality ofsections of a first of said two images, whereby to assign an imagequality to each of said plurality of sections of the first image; for atleast one section of the first image that is determined to have an imagequality different from the image quality of others of said plurality ofsections of the first image; perform an image processing process, theimage processing process comprising: configuring a matching algorithmfor that section of the first image in dependence on the assigned imagequality of the section; and using the configured matching algorithm tocompare said section of the first image with a corresponding section ofthe second of said two images whereby to generate an output; and, usesaid output to determine whether the first and second images representthe same entity.

According to a sixth aspect of the present invention, there is provideda computer program for use in verifying whether a user of a devicecorresponds to a previously authenticated user, the computer programcomprising instructions such that when the computer program is executedon a processing system having access to a first image and a secondimage, the processing system is configured to carry out a methodaccording to the third aspect, wherein: the first image is an image ofan identity document comprising an image of the previously authenticateduser and data identifying the previously authenticated user, theidentity document having been validated in respect of the previouslyauthenticated user by a trusted authority, whereby to previouslyauthenticate the user, and the processing system is arranged to derive,from the identity document, said data identifying the previouslyauthenticated user, and the second image is an image captured by thedevice.

According to a seventh aspect of the present invention, there isprovided a method of comparing two images by a processing system inorder to determine whether they represent the same entity, the methodcomprising: assessing an image quality of each of a plurality ofsections of a first of said two images, whereby to assign an imagequality to each of said plurality of sections of the first image; for atleast one section of the first image that is determined to have an imagequality different from the image quality of others of said plurality ofsections of the first image; performing an image processing process, theimage processing process comprising: configuring a matching algorithmfor that section of the first image in dependence on the assigned imagequality of the section; and using the configured matching algorithm tocompare said section of the first image with a corresponding section ofthe second of said two images whereby to generate an output; using saidoutput to determine whether the first and second images represent thesame entity.

The quality of a section of the first image can be usefully assessed bydetermining the amount of detail the section contains that is useful forperforming an image matching process with another image. Sections of thefirst image that contain little in the way of detail that is useful forfacial matching can bias the overall comparison result between the firstimage and the second image. By considering the image quality of thefirst image section-by-section, the sections of the first image thatcontain little such detail can be identified, and can be taken intoaccount when configuring the matching algorithm, such that the biasingeffect of those sections can be reduced.

In one arrangement, the image quality of a said section is determined byidentifying features within that section and comparing characteristicsof said features to characteristics of features of a predetermined setof training features. The set of training images may comprise one ormore sets of images with “desired” features and one or more sets ofimages with “undesirable” features. A set of images with desiredfeatures may be made up of images that are suitable for imagecomparison. Sets of images having undesirable features may include, forexample, sets of images with poor lighting, or blurred features.

Alternatively, or additionally, the image quality of a said section maybe determined by identifying features within that section anddetermining the sharpness of the identified features. A section havingfeatures with a relatively high determined sharpness may be assigned ahigher image quality than a section having features with a relativelylow determined sharpness. The sharpness of a feature can be assessed by,for example, determining the change in pixel intensity over a givenarea. A large change in pixel intensity over a relatively small areacould indicate a relatively sharp feature, whereas a smaller change overa larger area could indicate a relatively unclear feature. Sections ofan image that contain sharp features typically contain detail that canbe useful for image matching.

In one arrangement, the matching algorithm for a given section of thefirst image is configured to: convert said section of the first imageand the corresponding section of the second image into first and secondnumerical representations of said sections of the first and secondimages respectively, the first and second numerical representationsbeing representations of characteristics of said features within thesections of the first and second images respectively; and, compare saidfirst and second numerical representations whereby to determine whetherthe first and second images represent the same entity.

The method may, in one particular arrangement, comprise performing theimage processing process for a plurality of said sections of the firstimage, whereby to generate a plurality of outputs, each corresponding toa comparison for a said section of the first image; and using therespective outputs for said sections of the first image to determinewhether the first and second images represent the same entity.

Additionally, the configured matching algorithm may be configured tocombine said respective outputs, said combining comprising: assigning arelatively high weighting to an output corresponding to a comparison fora said section of the first image having a relatively high assignedimage quality; assigning a relatively low weighting to an outputcorresponding to a comparison for a said section of the first imagehaving a relatively low assigned image quality; and combining saidweighted outputs.

In the present arrangement, said weighted outputs may be combined togive a value indicative of the probability that the first and secondimages represent the same entity, and said step of determining whetherthe first and second images represent the same entity may comprisecomparing said value to a predetermined threshold value.

Optionally, the first image may be an image that has been captured by orin conjunction with the processing system. Alternatively oradditionally, the second image may be an image that has been captured byor in conjunction with the processing system. Where the processingsystem is a component of a user device, such as a mobile device, thefirst and/or second image may be captured by an image capture componentof the user device, for example. Alternatively, where the processingsystem is, for example, a remote server, the first and/or second imagemay have been captured by a device remote from the processing system,which is working in conjunction with the processing system to determinewhether the first and second images represent the same entity.

In the event that it is determined that the first and second images donot represent the same entity, the method may comprise successivelycapturing further images by or in conjunction with the processingsystem, and comparing each said further image with the first imagewhereby to determine whether they represent the same entity. Thisarrangement is particularly advantageous where the second image is animage captured by a mobile device, because the image capture conditionsof a mobile device can vary (the lighting, for example, depends greatlyon the location and orientation of the device). Thus, if it wereerroneously determined that the first and second images do not representthe same entity due to the poor image quality of the second image, afurther image can be captured and compared to the first image. Thelikelihood of the comparison result being correct for the further imageof the user can be increased if the further image is of suitablyimproved image quality.

In one arrangement, the method further comprises comparing the overallimage qualities of the two images and designating the image with thelower overall image quality as the first image and the image with thehigher overall image quality as the second image. The lower-qualityimage is likely to have a greater number of sections that contain littlein the way of detail that is useful for image comparison, and thus theeffectiveness of the above method in improving the reliability of theimage matching result can be increased be designating the lower-qualityimage as the first image.

According to an eighth aspect of the present invention, there isprovided a processing system for use in comparing two images in order todetermine whether they represent the same entity, the processing systembeing configured to: assess an image quality of each of a plurality ofsections of a first of said two images, whereby to assign an imagequality to each of said plurality of sections of the first image; for atleast one section of the first image that is determined to have an imagequality different from the image quality of others of said plurality ofsections of the first image; perform an image processing process, theimage processing process comprising: configuring a matching algorithmfor that section of the first image in dependence on the assigned imagequality of the section; and using the configured matching algorithm tocompare said section of the first image with a corresponding section ofthe second of said two images whereby to generate an output; and, usesaid output to determine whether the first and second images representthe same entity.

According to an ninth aspect of the present invention, there is provideda computer program for use in comparing two images in order to determinewhether they represent the same entity, the computer program comprisinginstructions such that when the computer program is executed on aprocessing system having access to two images, the processing system isconfigured to carry out a method according to the seventh aspect.

According to a tenth aspect of the present invention, there is provided,a method of verifying whether the user of a device is a user that hasbeen previously authenticated in respect of the user, in which thedevice has access to a plurality of images, at least two of which havebeen captured for the user within a continuous time period, the methodcomprising: performing a difference detecting process for said at leasttwo images, said difference detecting process comprising: comparing saidat least two images whereby to detect differences therebetween; and,determining whether said detected differences are sufficient to indicatethat said at least two images correspond to a live user, whereby tooutput a liveness indicator; and in dependence in the livenessindicator, selectively comparing one of said at least two images to apreviously validated image of said previously authenticated user in animage matching process in order to determine whether said compared imagecorresponds to the previously authenticated user.

Performing such a difference detecting process before comparing acaptured image for a user to a previously validated image of thepreviously authenticated user ensures that the captured image for theuser is an image of a live user (i.e. the user in possession of thedevice) and is not, for example, an image of a photograph of a user notin possession of the device. If the difference between the two images isnot found to be sufficient, then the image matching process may not beperformed and the user may not be verified as the previouslyauthenticated user.

In the event that said detected differences are determined not to besufficient to indicate that said at least two images correspond to alive user, the method comprises repeating said difference detectingprocess for a different two images which have been captured for the userwithin said continuous time period whereby to output a livenessindicator for said two different images, and selectively performing animage matching process for one of said different two images independence upon the liveness indicator. This is useful in the case thatthe user of the device is remaining particularly still as it allows moretime for movement of the user to be detected.

Furthermore, the method may further comprise repeating said differencedetecting process for a plurality of different images that have beencaptured within said continuous time window. Thus, in effect, the userof the device is given a predetermined time in which they can “prove”that they are a live user (i.e. they are given a predetermined time todisplay movements indicative of “liveness”). If the difference detectingprocess does not find sufficient difference between the images capturedin that time window, it may be determined that the images are not imagesof a live user, and thus the image matching process may not be carriedout for the user.

In one arrangement, said step of detecting differences between said atleast two images comprises comparing the pixels within a section of afirst of said at least two images to the pixels within a correspondingsection of the second of said two images, said sections being sectionsthat have been determined to include facial features. Changes in theexpression of the user can be detected in this way and such changes canbe used to identify an image of a live user.

Alternatively or additionally, said step of detecting differencesbetween said at least two images may comprise comparing the pixelswithin a section of a first of said at least two images to the pixelswithin a corresponding section of the second of said at least twoimages, said sections being sections that have been determined toinclude both facial features and background features. Changes of theposition of the user with respect to the background can be detected inthis way and such changes can be used to identify an image of a liveuser.

According to a eleventh aspect of the present invention, there isprovided a processing system for verifying whether the user of a deviceis a user that has been previously authenticated in respect of thedevice, the processing system having access to a plurality of images, atleast two of which have been captured by the device for the user withina continuous time period, the processing system being configured to:perform a difference detecting process for said at least two images,said difference detecting process comprising: comparing said at leasttwo images whereby to detect differences therebetween; and, determiningwhether said detected differences are sufficient to indicate that saidat least two images correspond to a live user, whereby to output aliveness indicator; and in dependence in the liveness indicator,selectively compare one of said at least two images to a previouslyvalidated image of said previously authenticated user in an imagematching process in order to determine whether said compared imagecorresponds to the previously authenticated user.

According to a twelfth aspect of the present invention, there isprovided a computer program for use in verifying whether the user of adevice is a user that has been previously authenticated in respect ofthe device, the computer program comprising instructions such that whenthe computer program is executed on a processing system having access toa plurality of images, at least two of which have been captured for theuser within a continuous time period, the processing system isconfigured to carry out a method according to the tenth aspect.

The processing system in any of the above aspects may comprise at leastone processor and at least one memory including computer programinstructions, the at least one memory and the computer programinstructions being configured, with the at least one processor, toperform the methods of the first, fourth, seventh and tenth aspectsdescribed above. The processing system may be embodied on a userterminal device such as a mobile device, while certain functionalitiesdescribed above may be embodied on a server system, in which case theimages can be received by the server system from a device remotetherefrom. Further, the inventions described herein may be embodied on anon-transitory computer-readable storage medium storing said computerprogram instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically an exemplary first captured imageaccording to an embodiment of the present invention;

FIG. 2 illustrates schematically an exemplary second captured imageaccording to an embodiment of the present invention;

FIG. 3 illustrates schematically an exemplary device configured to carryout a method according to an embodiment of the present invention;

FIG. 4 shows a flow chart of a method according to an embodiment of thepresent invention;

FIG. 5 illustrates schematically a close up of an exemplary firstcaptured image according to an embodiment of the present invention;

FIG. 6 illustrates schematically an exemplary device configured to carryout a method according to an embodiment of the present invention; and,

FIG. 7 an exemplary processing system, and exemplary devices configuredto carry out a method according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

A conventional way of verifying the identity and/or credentials of aperson is to ask that person to provide documentation that supportstheir identity and/or credentials. For example, a person may be asked toprovide a valid photographic ID, such as a passport or driving licenseas proof of their identity. In this case, in order to verify thatperson's identity, typically two separate checks are performed. Firstly,the validity of the photographic ID is checked and secondly the personproviding the photographic ID is compared to the image on thephotographic ID in order to verify that the photographic ID belongs tothat person. Typically, these checks are performed by a human

There are known techniques for checking the validity of an identitydocument, such as a photographic ID, via a device. For example, byconfiguring a device to look for certain features in an image, it ispossible to verify, up to a reasonable level of certainty, via a device,whether an image of an identity document is an image of a valid identitydocument. Such features may include, for example, the inclusion ofcertain check digits within machine readable zones on the identitydocument (which can be read by a device using optical characterrecognition (OCR) techniques), or the inclusion of an image of a humanface that is located in an expected position relative to other featuresof the document. Other validity indicators include, for example, theinclusion of water marks or holograms, and the use of particular fonts.

It has been recognised by the present inventors that, if it werepossible to perform the second check via a device, namely the comparisonof the face of a user of a device to the picture of a human face on aphotographic ID held by the user of the device, then it would bepossible to authenticate the user of the device in this way. It has beenrealised by the present inventors that this could be achieved byconfiguring a device to capture an image of the user of the device, andan image of an identity document held by the user of the device, and tocompare the image of the user of the device to the picture of the humanface on the identity document to determine whether they represent thesame entity. FIGS. 1 and 2 show examples of two such captured images100, 200.

The first image 100, as illustrated schematically in FIG. 1, is an imageof an identity document 110, which is associated with a person. Theidentity document 110 contains a picture 120 of the person associatedwith the identity document 110. Typically, an identity document 110 willinclude details 130 which can be used to identify the identity and/orother credentials of the person associated with the identity document110. Some identity documents 110 may also comprise a chip, which storesadditional information about the person associated with the identitydocument 110 and can be interrogated by a chip reader via a suitableprotocol. The chip may store, for example, biometric information, suchas a digital image of the person associated with the identity document110 and/or other identifying information about the person, e.g. name,address, etc., together with data relating to the authority that issuedthe identity document 110.

Identity documents are typically issued by a trusted authority, such asthe Government, for example. Such a trusted authority will havepreviously verified that the picture 120 is a picture of the personassociated with the identity document 110 and will have authenticatedthat person as the person associated with the details 130. The identitydocument may be a physical document, such as an identity card, passportor certificate, or it may be an electronic document, such as a digitalphotograph and associated identity data.

The second image 200, as illustrated schematically in FIG. 2, is animage of the user 210 of a device, which has been captured e.g. by acamera on the device. By comparing the first and second images 100, 200,it is possible to verify whether the user 210 of the device, at the timethat the second image 200 was captured, is the person associated withthe identity document 110.

Many facial recognition and matching technologies exist in the art. Inorder to reliably perform the required matching, most such technologiesrequire facial images to be of a high quality such that they containsufficient distinctive detail to determine whether they represent thesame person. Factors that typically affect the reliability of facialmatching between two images include the resolution of the images (whichcan be usefully quantified as the number of pixels between the eyes ofthe person) and the illumination of the person's face. Images with toomuch illumination appear washed out, such that only very strong facialfeatures, such as the eyes and nose stand out, and images with toolittle illumination have only very limited contrast and thus thementioned strong facial features are less visible.

Pictures 120 on identity documents 110 are typically of a low quality.For example, they are typically small, over-exposed and have a lowresolution. Furthermore, many identity documents 110 have visiblesecurity features printed over the picture 120, which can obscure facialdetails in the picture 120, making facial matching difficult. If theidentity document 110 is subsequently imaged, the quality of the face inquestion is yet reduced.

Current facial matching technologies do not perform well enough toreliably perform a comparison between the very low-quality capturedpicture 120 in an image 100 of an identity document 110 and an image 200of a user 210 of a device, which has been captured by the device.Aspects of the present invention are, therefore, concerned withproviding an image matching method that can reliably compare alow-quality image with another image so as to determine whether theyrepresent the same entity.

FIG. 3 shows a block diagram of a device 300 arranged to carry out acomparison according to an exemplary embodiment of the presentinvention. The device 300 may be, for example, a mobile phone, acomputer or a tablet. The device 300, in this example, comprises aprocessing system 310 and an image capture component 320, such as acamera. The image capture component 310 may be integral with the device300, or it may be separate from, but communicable with the device 300.

In the present arrangement, the device 300 is configured to capture botha first image 100 of an identity document 110 associated with apreviously authenticated user, and a second image 200 of a user 210 ofthe device 300. These images 100, 200 are provided to the processingsystem 310 as illustrated schematically by the arrows in FIG. 3. In analternative arrangement, the processing system 310 may be remote fromthe device 300, in which case, the device 300 may send the first andsecond images 100, 200 to the processing system 310 via a wired orwireless network for example. This arrangement is discussed in moredetail below, with reference to FIG. 7.

In yet another arrangement, the first image 100 may have previously beencaptured and stored in a storage device, and the processing system 310may be arranged to retrieve the first image 100 from the storage device.

The processing system 310 is arranged to compare the first image 100 tothe second image 200 to determine whether they represent the same user(i.e. to determine whether the user 210 represented in the second image200 is the previously authenticated user associated with the identitydocument 110). FIG. 4 shows a flow chart illustrating steps involved insuch a comparison process according to an embodiment of the presentinvention.

At step 400, the processing system 310 is configured to assess an imagequality of each of a plurality of sections of the first image 100,whereby to assign an image quality to each of the plurality of assessedsections of the first image 100. FIG. 5 shows a close-up of the firstimage 100, showing the captured picture 120 of a previouslyauthenticated user associated with the identity document 110. Twoexemplary sections 500, 510 of the image are indicated by dashed lines,the first 500 covering the eye area, and the second 510 covering thecheek area. In this example, each of these sections 500, 510 is assignedan image quality. The assigned image quality may correspond to thesuitability of that section for facial matching, which can be affectedby a number of factors as discussed above.

Typically, images are made up of an array of pixels having differentintensities. In one arrangement, the quality of a section, such as thefirst section 500, may be assessed using wavelet processing to identifythe variation in pixel intensity between pixels within a given areawithin that section.

More specifically, considering the first section 500 as an example, agrid of wavelets can be convolved with the pixels that make up the firstsection 500 whereby to provide responses indicative of the change inpixel intensity over the area covered by the wavelet. By using waveletsof differing sizes, features of the image can be identified and the“sharpness” of those features can be determined.

For example, a large change in pixel intensity over a relatively smallarea would indicate a relatively sharp feature, whereas a smaller changeover a larger area would indicate a relatively unclear feature. Sectionsof the image that contain sharp features typically contain detail thatcan be used for facial matching. For example, images of eyes, whichtypically contain a great deal of detail in a relatively small area,usually contain relatively large variations in pixel intensity over asmall region. Images of cheeks, on the other hand, which typicallycontain little in the way of detail that is useful for facial matching,usually contain very little variation in pixel intensity over the wholearea. Thus sections of the image 100 having sharper features (i.e. agreater variation in pixel intensity per unit area) may be assigned ahigher quality than sections with fewer sharp features. In this case,the first section 500 would most likely be assigned a higher imagequality than the second section 510.

Further, the sharpness of the features identified in a given section,and/or other characteristics of the identified features, may be comparedto the characteristics of features within a set of training images. Theset of training images may comprise one or more sets of images with“desired” features and one or more sets of images with “undesirable”features. These sets of images can be used to further assess the qualityof a section of an image 100. For example, where a section of the image100 having sharp features has been identified, the test images may beused to determine whether those sharp features are likely to be facialfeatures or whether they are likely to be non-facial features, such assecurity marks printed over the face, for example. A set of images withdesired features may be made up of a set of images of human faces thatare suitable for facial comparison. Sets of images having undesirablefeatures may include, for example, sets of images of human faces withfeatures such as security marks imposed thereon. Sets of training imagesmay also be used to train the processing system 310 to distinguishbetween the face of a person with light skin, which was captured in lowlighting conditions, and the face of a person with darker skin.

Through use of such training images, the processing system 310 can,therefore be trained to distinguish between the characteristics ofdesirable facial features and the characteristics of un-desirablefeatures. In other words, training images can be used to identify thesections of an image that are likely to be of most use when performingan image comparison. In this arrangement, sections 500, 510 that havebeen determined to include desirable features may be assigned a highimage quality relative to sections with fewer desirable features, and/orsections with more undesirable features.

As another example, the quality of a section may alternatively oradditionally be assessed by determining the number of pixels per unitarea in a given section. It will be appreciated that there are otherindicators of image quality, and these may be used as an alternative orin addition to the above to assign image qualities to sections of thefirst image 100.

After the processing system 310 has assigned image qualities to aplurality of sections of the first image 100, the processing system 310then performs an image processing process for at least one section ofthe first image 100 that is determined to have an image qualitydifferent from the image quality of the other assessed sections of thefirst image 100. The processing process comprises steps 410 and 420.Considering the first section 500 as an example, at step 410, theprocessing system 310 configures a matching algorithm for the firstsection 500 in dependence on the assigned image quality of that section500.

At step 420, the processing system 310 uses the configured matchingalgorithm to compare the first section 500 with a corresponding sectionof the second image 200 (i.e. a section of the second image 200 thatcovers the same part of the face as is covered by the first section 500of the first image 100).

The section of the second image 200 that corresponds to the firstsection 500 of the first image 100 may be determined using conventionalfacial recognition techniques such as those described above to identifythe major features of a human face, such as the eyes, nose, and mouth,in each of the first and second images 100, 200. These features may beused as anchor points to fit a grid of sections to each of the images100, 200 such that each section in the grid covers a predeterminedportion of the face.

The output from the matching algorithm for the first section 500 of thefirst image 100 may be indicative of the probability that the firstsection 500 represents a part of a face that is also present in thecorresponding section of the second image 200. The matching algorithmmay be configured to compare the first section 500 to the correspondingsection of the second image 200 by comparing the features (orcharacteristics of the features) within the first section 500 with thefeatures (or characteristics of the features) within the correspondingsection of the second image 200 to determine whether there is a match.

More specifically, in one arrangement, the processing system 310 maycompare the first section 500 of the first image 100 to thecorresponding section of the second image 200 by first analysing thevariation in pixel intensity as discussed above. The variation in thepixel intensity across the first section 500 may be representednumerically. This process may be repeated for the corresponding sectionof the second image 200 thereby generating a numeric representation ofthe change in pixel intensity across this section of the second image200. The two numeric representations may then be compared to determinewhether the two sections have the same feature characteristics.

To speed up the comparison process, and reduce the computational demandon the processing system 310, the size of the numerical representationsof the sections may, in one arrangement, be reduced using discriminantanalysis.

The image processing process may be repeated for multiple sections ofthe first image 100, in which case the matching algorithm is configuredaccording to the assigned image qualities of multiple sections of thefirst image 100 and is used to compare those sections of the first image100 to the corresponding sections of the second image 200 therebygenerating multiple outputs.

Finally, at step 430, the processing system 310 is configured to use theone or more outputs from the matching algorithm to determine whether thefirst image 100 includes an image of the user 210.

Assessing the image quality of sections of the first image 100 andconfiguring the matching algorithm in dependence upon the assigned imagequality of at least one of those sections means that the image qualitiesof the different sections can be taken into account when assessing thesignificance of a close match (or lack thereof) between a given one ofthose sections and the corresponding section of the second image 200.

In an example where the first section 500 of the first image 100 isassigned a higher image quality than the second section 510, greatersignificance may, for example, be attached to a close match (or lackthereof) between the first section 500 and the corresponding sectionthan would be attached to a close match (or lack thereof) between thesecond section 510 and the corresponding section of the second image200.

This is particularly useful where the first image 100 is of generallylow quality (as is typically the case with captured images of pictures120 on identity documents 110, as discussed above). This is because, ifthe first image 100 were to be compared to the second image 200 as awhole, strong correspondence (or lack thereof) between parts of thefirst image 100 that have little detail (such as the cheeks forexample), may bias the overall comparison result, leading to anincorrect determination as to whether the images 100, 200 represent thesame user 210. In other words, by considering the quality of the firstimage 100 section-by-section, a matching algorithm can be configuredthat takes into account the biasing effect of sections of an image thathave a low image quality.

In one particular arrangement, the matching algorithm may be configuredto take these biasing effects into account by weighting the outputs forthe sections of the first image 100 and then combining the weightedoutputs to generate a value indicative of the probability that the firstand second images 100, 200 represent the same user 210. The weightingsfor outputs for sections of the first image 100 with a higher imagequality can be set higher than the weightings for outputs for sectionswith a lower image quality. The combined weighted outputs may then becompared to a threshold value to determine whether the images representthe same user.

In one arrangement, the matching algorithm may be configured forsections of the first image that have been determined to have an imagequality above a predetermined threshold. In this case, those sectionswith assigned image qualities above that threshold are compared to thecorresponding sections of the second image 200, and the sections withassigned image qualities below the threshold are not compared to thesecond image 200. This reduces the computational demand on theprocessing system 310 and prevents strong similarities or differencesbetween those low-quality sections and the corresponding sections of thesecond image 200 from biasing the overall comparison result.

If it is determined that the first and second images 100, 200 are imagesof the same user 210, the user 210 of the device 300 can beauthenticated as the user associated with the identity document 110.Before the user 210 is authenticated as the user associated with theidentity document 110, the processing system 310 may carry out furthersteps to verify that the image 100 is an image of a valid identitydocument, as will be described in more detail below.

Although the above method for comparing two images to determine whetherthey represent the same user 210 has been described in the context ofcomparing a picture 120 in a first image 100 of an identity document 110to a second image 200 of a user 210 of a device 300, it will beappreciated that the method is applicable for comparing any two imagesto determine whether they represent the same entity. As discussed above,the method is particularly useful where the first image 100 is alow-quality image, such as any image that was previously captured on areprographic medium other than one directly associated with theprocessing system 310, as the biasing effects of the low-qualitysections of the image on the overall comparison result can be reduced.

In general, therefore, the method may comprise a preliminary step ofassessing the overall image qualities of the two images to be comparedand designating the image with the lower overall image quality as thefirst image 100 and the image with the higher overall image quality asthe second image 200 before carrying out the image matching process asdescribed above.

When it is known that the one of the two images to be compared is animage 100 of an identity document 110, it may be assumed that the imageof the identity document 110 is the lower-quality image (as discussedabove, the pictures 120 on identity documents 110 are, in general, ofvery poor quality for the purpose of facial matching).

More generally, however, where the above method is used to compare anytwo facial images, the image qualities may be assessed with respect tothe suitability of the images for facial comparison. Factors that affectthe suitability of an image of a person for facial comparison include:whether that person was stationary when the image was captured, whetherthe person was looking at the camera (or other image capture device)when the image was captured, whether the person had their eyes open, andwhether the person is wearing items that obscure their face, such asglasses. As mentioned above, other factors include the resolution of theimage and the illumination of the person's face.

In one arrangement, the above-mentioned set of training images may beused to assess the quality of the images 100, 200. The use of trainingimages to train the processing system 310 to recognise certain“desirable” features and to distinguish them from other similar“undesirable” features, as discussed above. For this purpose, trainingimages may be used to train the processing system 310 to recogniseimages where lighting is sub-optimal, for example. Thus, the processingsystem 310 may determine which of two images to be compared is the lowerquality image by determining which of those images has the most“desirable” features.

As an alternative or additional preliminary step, the processing system310 may compare the image quality of the two images to a thresholdquality and may, for example, request an alternative image if the imagequality of one of the images is below the threshold quality. It isparticularly useful to compare the quality of the second image 200 to athreshold quality, because a better second image of the user 210 may becaptured by, for example, instructing the user 210 of the device 300 tofind better lighting conditions.

In addition or as an alternative, in the event that the first image 100is determined not to be an image of the user 210 represented in thesecond image 200, the device 300 may be configured to capture a furtherimage of the user 210 and to compare this image to the first image 100as described above. Again, the user 210 may be given directions as tohow to improve the quality of the second image 200. Thus, if the firstimage 100 was actually an image of the user 210, but was determined notto be an image of the user 210 due to the poor image quality of thesecond image 200, then the likelihood of the comparison result beingcorrect for the further image of the user 210 can be increased bycapturing a further second image, of suitably improved image quality.

As mentioned above, before the user 210 is authenticated as the userassociated with the identity document 110, the processing system 310 maycarry out steps to verify that the image 100 is an image of a valididentity document. In one exemplary embodiment, the identity document110 may comprise a chip which stores data relating to the identity ofthe user associated with the identity document 110, and this data may beused to verify that the image 100 is an image of a valid identitydocument. The data may comprise, in particular, a digital image of theuser associated with the identity document 110 and/or other data for theuser, such as the name, address and/or date of birth of the userassociated with the identity document 110. Typically, this data will beencrypted within the chip.

In one arrangement, the device 300 may be configured to retrieve thedata from the chip and to pass this data to the processing system 310.The processing system may then use this data to validate the identitydocument 110. In effect, therefore, the processing system is configuredto derive data for use in validating the identity document from the chipof the identity document, via the device 300. Where the data isencrypted, the data may be decrypted by the processing system 310 beforeit is used to validate the identity document.

As a particular example, where the data stored in the chip of theidentity document 110 comprises a digital image of the user associatedwith the identity document 110, the processing system 310 may beconfigured to compare the digital image from the chip to the first image100. By this method, the processing system 310 is able to determinewhether the picture 120 on the identity document 110 has been tamperedwith (e.g. replaced with a picture of a different user). If the firstimage 100 and the image derived from the chip are determined torepresent the same user, then the processing system may determine thatthe identity document 110 in the first image 100 is valid.

Alternatively, or additionally, the digital image derived from the chipof the identity document 110 may be compared to the second image 200(i.e. the image 200 of the user 210 of the device 300). This comparisonmay be performed in place of the comparison between the first and secondimages 100, 200 described above, or in addition to the comparisonbetween the first and second images 100, 200. When such a comparison isperformed in addition to the comparison between the first and secondimages 100,200, this can improve the reliability of the userauthentication method. In a specific arrangement, the comparison resultfor the second image 200 and the first image 100 may be combined withthe comparison result for the second image 200 and the image derivedfrom the chip. The combined result may be used to determine whether theuser 210 of the device 300 is likely to be the user associated with theidentity document 110.

In one arrangement, the data stored in the chip of the identity document110 may be retrieved using near field communication (NFC). In such anarrangement, the device 300 may comprise an NFC reader component whichis configured to retrieve the data stored in the chip when in closeproximity to the chip. Alternatively, the device 300 may becommunicatively connected to a separate NFC reader via, for example, aUSB port.

In one specific embodiment the identity document 110 may be anelectronic Machine Readable Travel Document (eMRtd), or a similaridentity document 110 which complies with the ICAO (International CivilAviation Authority) eMRtd standards. Such identity documents comprise achip, which can be used, inter alia, to verify the validity of theidentity document 110. There are a number of methods by which thevalidity of such an identity document 110 can be verified using thechip, as will be detailed below. However, first, an eMRtd, or a similaridentity document 110 which complies with the ICAO eMRtd standards willbe described in more detail.

The chip of an eMRtd stores first data in a “Logical Data Structure”.The first data may include for example data corresponding to datavisible on the surface of the identity document 110. As a specificexample, the first data may comprise data corresponding to data that isencoded in optical character recognition (OCR) format in a machinereadable zone (MRZ) of the identity document 110.

The chip also stores a “Document Security Object”, which is for use inverifying the validity of the identity document. The Document SecurityObject comprises a hash of the first data. It may also comprise a publickey of the identity document 110, as will be described in more detailbelow.

The Document Security Object is signed by the issuing authority; that isto say, the Document Security Object is encrypted with a private key ofthe issuing authority. The issuing authority may be, for example, agovernment.

In order to verify the validity of such an identity document 110, thedevice 300 may be configured to read the first data and the DocumentSecurity Object from the chip of the identity document 110. The data maybe read e.g. via a chip reader, such as a near field communicationreader, that is integral with, or connected to the device 300. This datamay then be sent to the processing system 310. Upon receipt, theprocessing system 310 may be configured to identify the authority whoissued the identity document 110 and obtain their public key.

The issuing authority can be identified from data derived from theidentity document 110. For example, the issuing authority may beidentified by data encoded in a Machine Readable Zone of the identitydocument 110. In this case, the processing system 310 may be configuredto analyse the first image 100 and extract the data identifying theissuing authority using Optical Character Recognition techniques, forexample.

Having identified the issuing authority, the public key may then beobtained, for example, from a Public Key Directory maintained by atrusted third party.

Alternatively, the public key may be stored on the chip, and may havebeen read by the device 300 and sent to the processing system 310,together with the first data and the Document Security Object.

Alternatively, the processing system 310 may have been pre-configuredwith the public key for the issuing authority.

Irrespective of how the public key is retrieved, the processing system310 may be configured to verify the validity of the identity document110 by first decrypting the Document Security Object using the publickey of the issuing authority. The processing system 310 can therebyverify that the Document Security Object is a valid Document SecurityObject.

Once decrypted, the processing system 310 may be configured to comparethe decrypted Document Security Object to a hash of the first data. Ifthere is correspondence, the processing system 310 can verify that thefirst data has not been tampered with, and that the identity document110 is valid.

In addition to the above, the data stored in the chip (i.e. the firstdata, the Document Security Object, and any other data stored thereon)may be encoded. In one specific embodiment, the data may have beenencoded using a key derived from data that is visible on the surface ofthe identity document. Such visible data could include, for example,data encoded in an OCR format in a MRZ of the identity document 110.

Thus, in order to read the first data and the Document Security Objectfrom the chip of the identity document 110, the device 300 may first berequired to derive the visible data from the surface of the identitydocument. This data could be derived directly from the surface of theidentity document 110, or from the first image of the identity document100, e.g. using OCR techniques.

If the processing system 310 is successfully able to decode the datastored on the chip using the data visible on the surface of the identitydocument 110, it can be determined that the chip of the identitydocument 110 has not been replaced, and/or that the visible data on thesurface of the identity document 110 has not been altered.

The chip may further comprise a secure element that contains a privatekey for the identity document 110. In this case the device may send achallenge to the chip, which causes the chip to respond with a responsethat is signed with the private key of the identity document 110.

Upon receipt of the signed response, the processing system 310 may beconfigured to verify, using the public key of the identity document 110that the response has been signed by the private key of the identitydocument 110. This provides assurance that the data stored on the chipof the identity document 110 has not been duplicated from another chip.

As will be appreciated, where the processing system 310 is a componentof the device 300, the validity of the identity document will beverified by the device 300 itself. Where the processing system 310 isremote from the device 300, the verification will be carried outremotely, and the device 300 is configured to send the data derived fromthe identity document 110 which is required by the processing system 310to verify the validity of the identity document 110 to the processingsystem 310.

Alternatively or additionally to the above-described validity checks,which make use of data stored on a chip of the identity document 110and/or data visible on the surface of the identity document 110,validity checks may be carried out through use of data that is stored ina storage device remote from the identity document 110. As an example,said stored data may comprise an image of the user associated with theidentity document 110, and this image may be retrieved from the remotestorage device, and compared to one or both of the first and secondimages 100, 200 whereby to verify whether the identity document 110 isvalid and to improve the reliability of the user authentication result.Such a remote storage device could be, for example, a storage deviceheld by a government body, which stores validated images of citizens.

In one specific example, the image stored by the remote storage devicemay be retrieved through use of data derived from the identity document110 which uniquely identifies the user associated with the identitydocument 110. In other words, the image may be retrieved through use ofa unique user identifier derived from the identity document. Such aunique identifier could include, for example, a unique useridentification code, such as a passport number or a national insurancenumber and could be derived from the surface of the identity document110 and/or a chip of the identity document 110, as described above.

In the present example, the image stored by the remote storage devicemay be retrieved by first sending the derived unique user identifier tothe remote storage device whereby to identify the user associated withthe identity document 110 to the remote storage device. The remotestorage device may then use the unique user identifier to retrieve theimage of the user of the identity document 110 and may send theretrieved image to the device 300 and/or the processing system 310.

According to another aspect of the present invention, in one embodiment,the processing system 310 has access to a storage device 600, as shownin FIG. 6. Once it has been determined that the first image 100 of theidentity document 110 and the second image 200 of the user 210 of thedevice 300 represent the same user, the second image 200 of the user 210may be stored as a validated image of the user associated with theidentity document 110 in the storage device 600 as shown in FIG. 6.

As discussed above, typically, the second image 200 captured by thedevice 300 will be of a higher quality than the first image 100 of theidentity document 110. In one arrangement, if a user of the device 300later wishes to authenticate himself on the device 300 as the previouslyauthenticated user 210 associated with the identity document 110, theprocessing system 310 may capture a subsequent image 200* of the user ofthe device 300 and may compare the subsequent image 200* with thevalidated second image 200 to determine whether they are images of thesame user. The images may be compared according to the method describedabove, or alternatively, a conventional facial matching algorithm may beused to compare the images.

In the event that it is determined that the two images 200, 200*represent the same user, the processing system 310 can authenticate theuser of the device 300 as the previously authenticated user 210associated with the identity document 110.

In the present embodiment, therefore, once the user 210 has beenauthenticated once using an image 100 of an identity document 110, theuser 210 need not provide any further images of identity documents 100in order to authenticate himself in subsequent authentication events.Instead, the user 210 can authenticate himself using the storedvalidated image 200.

Furthermore, by storing the second image 200, in preference to, forexample, the first image 100, the reliability of subsequentauthentication events can be improved. This is because the second image200 will typically be of higher quality than the first image 100 andthus subsequent authentication events are carried out by comparing tworelatively high-quality images 200, 200* to one another, rather thancomparing a very low quality image (the first image 100) to a higherquality image 200*. In effect, therefore, it could be said that thesecond image 200 is designated as the higher quality image when it isstored. The validated second image 200 may be used for all subsequentauthentication events for the user 210.

Alternatively, in one arrangement, if it is determined that asubsequently captured image 200* of a user of the device 300 representsthe previously authenticated user 210 represented in the validatedsecond image 200, the processing system 310 may also store thesubsequently captured image 200* as a validated image of the previouslyauthenticated user 210 in the storage device 600. Two exemplarysubsequently captured images 200**, 200***, which have been previouslyvalidated using the second image 200 as being images of the userassociated with the identity document 110, are shown schematically asbeing stored in the storage device 600 in FIG. 6.

The processing system 310 may, in one arrangement, compare the qualitiesof the validated second image 200 and the subsequently captured image200* and may designate one as the higher quality image. Thereafter, in afurther subsequent authentication event, the processing system 310 mayselect the designated higher-quality image from the storage device 600and use that image in the further subsequent authentication event,thereby further improving the reliability of the comparison result inthe further subsequent authentication event. In one arrangement, theprocessing system 310 may assign an image quality to each storedvalidated image, and each time the processing system 300 validates auser of the device 300, it may select the highest quality validatedimage from the storage device 600 to authenticate the user.

The stored validated images 200, 200**, 200*** may be encoded with aone-way encoding algorithm before they are stored. In other words, theimages 200, 200**, 200*** may be stored as numerical representations,from which the original images cannot be derived. When a subsequentlycaptured image 200* of a user of the device 300 is compared to a storedencoded image 200, 200**, 200*** in the storage device 600, thesubsequently captured image 200* is therefore similarly encoded beforeit is compared to the stored encoded image. As discussed above, thecomputational demand on the processing system 310 is lower when acomparison is performed between numerical representations of images(e.g. encoded images), rather than between the original imagesthemselves, and thus by encoding images before they are compared, thecomparison can be carried out faster.

The processing system 310 may, in one arrangement, generate or otherwisederive a unique user identifier 610 for the user 210 associated with theidentity document 110, and may store this identifier 610 together withthe second image 200 and any other stored validated images 200**, 200***of the user 210. The processing system 310 may use this identifier 610to retrieve a validated image of the user 210 from the storage device600 in subsequent authentication events for the user 210.

In one particular example, the unique user identifier 610 for the user210 may be a hash value derived from details relating to the user 210.The details may, for example comprise the first and last name of theuser 210, and the date of birth of the user 210. These details may havebeen derived by the processing system 310 from the identity document 110(using optical recognition, for example, or other suitable techniques).

Thereafter, in order to identify the user 210 to the processing system310 in a subsequent authentication event for the user 210, theprocessing system 310 need only be provided with details relating to theuser 210 that is being authenticated, from which it can then derive theunique user identifier 610. The processing system 310 may alternativelyor additionally store certain user details in association with, butseparately from, the unique user identifier.

The processing system 310 may also, in one example, send the unique useridentifier 610 for the user 210 to a server remote from the processingsystem 310 whereby to indicate to the server that the user 210 of thedevice 300 has been authenticated as a user associated with the uniqueuser identifier 610. This is useful, for example, where the user of thedevice 300 is requesting access to a service provided by the remoteserver via the device 300, and the remote server needs to verify theidentity of the user 210 of the device 300 before it provides theservice.

In one arrangement, the processing system 310 may store details relatingto the identity of the user 210 associated with the identity document110 together with the validated image 200 of the user 210 and/or theuser identifier 610. These details may, in one example, have beenderived from the image 100 of the identity document 110. For example,where the identity document contains details 130 printed or otherwisepresented on a surface of the identity document 110 in text form, thesedetails may be extracted using optical character recognition and stored.

Additionally or alternatively, the details may have been derived fromdata stored in a chip of the identity document 110. More specifically,as mentioned above, an identity document 110 may comprise a chip whichstores data that identifies the user associated with the identitydocument 110 (e.g. the name, address and/or a digital image of theuser). The device 300 may be configured to retrieve the data from thechip (using NFC, for example), and pass this data to the processingsystem 310 to be stored. In other words, the processing system 310 maystore data which has been derived from a chip of the identity document110 via the device 300.

Additionally or alternatively, some or all of the stored details mayhave been retrieved from a remote storage device using data derived fromthe identity document. More specifically, the processing system 310 may,in one arrangement, be configured to derive data from the identitydocument 110 which uniquely identifies the user associated with theidentity document 110. In other words, the processing system 310 may beconfigured to derive a unique user identifier from the identity document110. The processing system 310 may then send the unique user identifierto the remote storage device, and the remote storage device may use theunique user identifier to retrieve details relating to the userassociated with the identity document 110, and to send the retrieveddetails to the processing system 310.

Additionally or alternatively, the stored details may have been providedby the user 210 of the device 300 when the user 210 was initiallyauthenticated with the first image 100 of the identity document 110.

In addition, or as an alternative, to storing the data derived from theidentity document locally at the processing system 310, the processingsystem 310 may arrange for the details relating to the identity of theuser 210 to be stored in a storage device 600 remote from the processingsystem 310, together with the validated image 200 of the user 210 and/orthe identifier for the user 210. The remote storage device 600 could bea storage device of a service provider with which the user is trying toauthenticate himself, for example.

In any event, in a subsequent authentication event, when a user of thedevice 300 is authenticated as the previously authenticated user 210associated with the identity document 110, these details can beretrieved from the storage device 600. The details may, in one example,be sent to a server remote from the processing system 310 whereby toidentify the authenticated user to the server.

As mentioned above with reference to FIG. 3, embodiments may be used toauthenticate users on a plurality of devices. When a captured image 200*of a user of a given device is determined to represent a previouslyauthenticated user 210 (either by comparison of the captured image 200*with an image 100 of an identity document 110 or by comparison of thecaptured image 200* with a stored image 200, 200**, 200***, which hasbeen previously validated by the processing system 310 as being an imageof the previously authenticated user 210), the processing system 310 maystore a unique device identifier for that device, together with thecaptured image 200*. This unique device identifier could be used inseveral ways as will become apparent in the following description.

The unique device identifier may be used to identify suspicious userbehaviour. As an example, if a user of a device tries to authenticatehimself as a given previously authenticated user, but the givenpreviously authenticated user has only ever authenticated himself on adifferent device, then the processing system 310 may be able todetermine, from the unique device identifiers for the devices, that theauthentication is a suspicious authentication.

Further, when a user of a device wishes to authenticate himself to aserver remote from the processing system 310 via the device, the uniqueuser identifier can be sent to a server whereby to identify to theserver on which device the user has been authenticated.

According to a further aspect of the present invention, before an image200 of the user 210 of the device 300 that has been captured by thedevice 300 is compared to an image of a previously authenticated user(i.e. either an image of an identity document 110 associated with theuser, or an image that has been previously validated by the processingsystem 310 as an image of the user), a check may be made to verify thatthe second image 200 is an image of an actual person (a “live” user)rather than e.g. a static photograph of the person.

Such a check may comprise the steps of capturing a series of images ofthe user of the device 300, and comparing successive images to look fordifferences between successive images that indicate that the images areimages of a live user. Once two successive images have been taken thatare sufficiently different to indicate that the images are images of alive user, the processing system 310 may use one of those images as thesecond image 200 in a comparison process as described above.

Carrying out such a check would prevent a user of a device 300 fromauthenticating himself as a different user by holding a photograph ofthe different user in front of the image capture component 320.

In one arrangement, before performing a comparison between twosuccessively captured images to look for differences between thoseimages, the images may be analyzed to determine portions of the imagethat represent a human face, and portions of the image that representbackground (sets of training images may be used for such an analysis, asdescribed above). In this arrangement, at least one section of one ofthe images that is determined to include both facial features andbackground features may be compared to a corresponding section of theother image to look for movement of the face with respect to thebackground. Such a comparison may be done on a pixel-by-pixel basis.

Alternatively or additionally, at least one section of one of the imagesthat is determined to include facial features only is compared to acorresponding section of the other image. Such a comparison may look fordifferences between the images indicative of facial movement, such asblinking.

Pairs of subsequently captured images may be compared either until apair of subsequently captured images is identified which aresufficiently different as to indicate the images are images of a liveuser, or until a predetermined number of pairs of subsequently capturedimages have been compared. Alternatively, pairs of subsequently capturedimages may be compared until a predetermined time has elapsed.

As mentioned above, with reference to FIG. 3 in particular, in onearrangement, at least part of the processing system 310 may be remotefrom the device 300. FIG. 7 shows schematically an exemplary remoteprocessing system 310 in such an arrangement. The processing system 310is communicatively connected to a plurality of devices, there being two(300 and 300*) such devices shown in FIG. 7.

In one example, a user of a first of the two devices 300 initiates auser authentication event on the first device 300 whereby to cause thefirst device 300 to capture an image of the user of the first device300. The first device 300 may also capture an image 100 of an identitydocument 110 associated with a user 210, as discussed above. In thisarrangement, the first device 300 then sends the two captured images100, 200 to the processing system 310, and upon receipt, the processingsystem 310 determines whether the two images 100, 200 are images of thesame user. The processing system 310 may carry out the steps as shown inFIG. 4 to determine whether the images represent the same user.

The first device 300 may optionally also retrieve data from a chip ofthe identity document 110 and may send this retrieved data to theprocessing system 310.

The authentication event may be associated with an authentication eventidentifier. The identifier may be generated by the processing system 310or the first device 300, but in any event, the authentication eventidentifier is shared between the two components 300, 310 whereby toidentify the authentication event to the two components 300, 310.

Once the processing system 310 has determined whether the imagesrepresent the same user, the processing system 310 may send anindication to the first device 300, so as to confirm the result of theauthentication event, together with the authentication event identifier,whereby to indicate to the first device 300 whether the user of thefirst device 300 is the user 210 represented in the identity document110 for that authentication event.

In the arrangement where the first device 300 sends data retrieved froma chip of the identity document 110 to the processing system 310, beforeconfirming the result of the authentication event to the first device300, the processing system 310 may use the data retrieved from the chipto perform further checks. In particular, where the data retrieved fromthe chip comprises an image of the user associated with the identitydocument 110, the processing system may compare this image to one orboth of the first and second images 100, 200 as described above. This isuseful both in verifying the validity of the identity document 110, andalso in increasing the reliability of the authentication result.

In an alternative arrangement, where the processing system 310 haspreviously authenticated the user 210, the processing system 310 mayalready have one or more validated images 200**, 200*** of the user 210stored in a storage device 600. In this case, therefore, the firstdevice 300 may not send an image 100 of an identity document 110associated with the user 210 to the processing system 310, but mayinstead send details identifying the user to the processing system 210which can be used by the processing system 310 to identify the user 210and retrieve a validated image of the user 210 from the storage device600.

As discussed above, in one arrangement, the processing system 310 maystore validated images 200**, 200*** of the user 210 together with auser identifier 610 for the user 210. In this arrangement, the detailssent from the first device 300 to the processing system 310 may comprisethe user identifier 610 for the user 210, or alternatively, the detailsmay comprise details from which the user identifier 610 can be derived.This latter case is applicable, for example, where the user identifier610 is a hash value as discussed above with reference to FIG. 6.

Once an validated image 200**, 200*** of the user 210 has been retrievedfrom the storage device 600, the processing system 310 compares theimage 200 of the user of the first device 300, which was received fromthe first device 300, to a previously validated image 200**, 200*** ofthe user 210 whereby to verify whether the user of the device 300 is thepreviously authenticated user 210.

Again, the authentication event may be associated with an authenticationevent identifier, and the processing system 310 may indicate theauthentication result, together with the authentication eventidentifier, to the first device 300.

As will be appreciated, users typically have more than one device, eachof which has the means for capturing images. Accordingly, the image 100of the identity document 110, may be captured by the second device 300*,while the first device 300 is used to take an image of the “live” user.This may be useful if, for example, the second device 300* can captureimages that are of a higher quality than the images captured by thefirst device 300. In this arrangement the authentication eventidentifier described above may be provided to both devices 300, 300* sothat the processing system 310 can identify that images received fromthe two different devices relate to the same authentication event.

Upon receiving the two images 100, 200, the processing system may beconfigured to verify that the two images 100, 200, are associated withthe same authentication event identifier before comparing them wherebyto determine whether they represent the same user, in the mannerdescribed above.

As previously mentioned, a given validated image 200 of a previouslyauthenticated user 210 may be stored in conjunction with detailsrelating to the device that is used to capture the image on which thepreviously authenticated user 210 validates himself. Thus, where apreviously authenticated user 210 has a plurality of devices 300, 300*,and authenticates himself via the plurality of devices 300,300*, aplurality of validated images 200**, 200*** of the user 210 may bestored in a remote storage device 600.

In one arrangement, in a subsequent authentication event for thepreviously authenticated user 210, the processing system 310 may selecta previously validated image 200**, 200*** of the previouslyauthenticated user 210 from the storage device 600 at least independence on the unique device identifier of the device on which thepreviously authenticated user 210 wishes to authenticate himself (i.e.the “authenticating” device). As an example, the processing system 310may select a previously validated image 200**, 200*** of the user 210that was captured by the authenticating device to validate the user ofthe authenticating device. This may improve the reliability of thefacial matching results, because the two images to be compared arelikely to be similar, as they were captured by the same device. Theprocessing system 310 may also determine which of the stored previouslyvalidated images 200**, 200*** to use when validating the user independence upon the designated image qualities of the images, asdiscussed above. For example, the processing system 310 may use apreviously validated image 200**, 200*** that was captured by a devicedifferent from the authenticating device if it is of significantlyhigher quality than a validated image that was captured by theauthenticating device.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. Forexample, in relation to the aspect of the invention where validatedimages are stored in a storage device 600, the processing system 310 maybe configured to assess the image qualities of each validated image andmay store an association between those images and their determined imagequalities. In a subsequent authentication event, the processing system310 may select the highest quality image from the storage device 600 andcompare this to an image of the user of a device whereby to authenticatethat user. Alternatively, the processing system 310 may only store acaptured image if it is of higher quality than the validated image of apreviously authenticated user with which it was compared. If thecaptured image is of higher quality, the processing system 310 mayreplace the previously validated image with the captured image, suchthat only one validated image of a given user is stored at any one time.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

Although at least some aspects of the embodiments described herein withreference to the drawings comprise computer processes performed inprocessing systems or processors, the invention also extends to computerprograms, particularly computer programs on or in a carrier, adapted forputting the invention into practice. The program may be in the form ofnon-transitory source code, object code, a code intermediate source andobject code such as in partially compiled form, or in any othernon-transitory form suitable for use in the implementation of processesaccording to the invention. The carrier may be any entity or devicecapable of carrying the program. For example, the carrier may comprise astorage medium, such as a solid-state drive (SSD) or othersemiconductor-based RAM; a ROM, for example a CD ROM or a semiconductorROM; a magnetic recording medium, for example a floppy disk or harddisk; optical memory devices in general; etc.

It will be understood that the processing system referred to herein mayin practice be provided by a single chip or integrated circuit or pluralchips or integrated circuits, optionally provided as a chipset, anapplication-specific integrated circuit (ASIC), field-programmable gatearray (FPGA), digital signal processor (DSP), etc. The chip or chips maycomprise circuitry (as well as possibly firmware) for embodying at leastone or more of a data processor or processors, a digital signalprocessor or processors, baseband circuitry and radio frequencycircuitry, which are configurable so as to operate in accordance withthe exemplary embodiments. In this regard, the exemplary embodiments maybe implemented at least in part by computer software stored in(non-transitory) memory and executable by the processor, or by hardware,or by a combination of tangibly stored software and hardware (andtangibly stored firmware).

What is claimed is:
 1. A method of determining whether a user of a mobile device corresponds to a previously authenticated user, the previously authenticated user having been previously authenticated via an identity document comprising at least one image of the previously authenticated user, wherein the at least one image comprises at least one of a portion of the identity document containing a photographic image of the previously authenticated user visible on the identity document and data representative of a digital image of the previously authenticated user stored on an integrated circuit component of the identity document, the method comprising: acquiring, by a processing system of the mobile device, a first image from the identity document, the first image being one of said photographic image and said digital image; causing, by the processing system of the mobile device, a camera connected to or integral with the mobile device to capture a second image, the second image corresponding to a user of the mobile device; comparing, by the processing system of the mobile device, data indicative of the first image and data indicative of the second image, whereby to determine whether the first image and the second image represent the same user; and responsive to determining that the first image and the second image represent the same user, forming, by the processing system of the mobile device, an association between the previously authenticated user and the mobile device.
 2. The method of claim 1, wherein forming the association comprises storing an association between a unique device identifier for the mobile device and data that uniquely identifies the previously authenticated user.
 3. The method of claim 2, wherein the data that uniquely identifies the previously authenticated user comprises the data representative of a digital image of the previously authenticated user.
 4. The method of claim 2, wherein the data that uniquely identifies the previously authenticated user comprises the data indicative of the second image.
 5. The method of claim 2, further comprising, responsive to the determining that the first image and the second image represent the same user, sending the unique device identifier to a remote server together with an indication that the user of the device has been verified by the processing system of the mobile device.
 6. The method of claim 5, further comprising: responsively authenticating, by the remote server, the previously authenticated user.
 7. The method of claim 5, further comprising: using the unique device identifier to identify, by the remote server, the device on which the previously authenticated user has been authenticated.
 8. The method of claim 1, wherein the previously authenticated user is associated with a unique user identifier, and responsive to the determining that first image and the second image represent the same user, the method further comprises storing, by the processing system of the mobile device, an association between the unique user identifier and the second image.
 9. The method of claim 8, wherein, responsive to the determining that the first image and the second image represent the same user, the method further comprises sending the unique user identifier to a remote server whereby to indicate that the user of the mobile device has been verified by the processing system of the mobile device as the user associated with the identifier.
 10. The method of claim 1, wherein the acquiring, by a processing system of the mobile device, the first image from the identity document comprises causing, by the processing system of the mobile device, the camera to capture the first image, the first image corresponding to an image of a portion of the identity document containing a photographic image of the previously authenticated user visible on the identity document.
 11. The method of claim 1, wherein the identity document comprises an integrated circuit component storing data representative of the digital image, and wherein the acquiring, by the processing system of the mobile device, the first image from the identity document comprises causing a chip reader connected to or integral with the mobile device to access the integrated circuit component, whereby to retrieve the data representative of the digital image of the previously authenticated user.
 12. A mobile device for use in determining whether a user of the mobile device corresponds to a previously authenticated user, the previously authenticated user having been previously authenticated via an identity document comprising at least one image of the previously authenticated user, wherein the at least one image comprises at least one of a portion of the identity document containing a photographic image of the previously authenticated user visible on the identity document and data representative of a digital image of the previously authenticated user stored on an integrated circuit component of the identity document, the mobile device comprising: at least one processor; and at least one memory including computer program code; wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the mobile device at least to: acquire a first image from the identity document, the first image being one of said photographic image and said digital image; cause a camera connected to or integral with the mobile device to capture a second image, the second image corresponding to a user of the mobile device; compare data indicative of the first image and data indicative of the second image, whereby to determine whether the first image and the second image represent the same user; and responsive to determining that the first image and the second image represent the same user, store an association between a unique device identifier for the mobile device and data that uniquely identifies the previously authenticated user.
 13. The mobile device of claim 12, wherein the data that uniquely identifies the previously authenticated user comprises the data representative of a digital image of the previously authenticated user.
 14. A mobile device of claim 12, wherein the data that uniquely identifies the previously authenticated user comprises the data indicative of the second image.
 15. The mobile device of claim 12, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the mobile device to, responsive to determining that the first image and the second image represent the same user, send the unique device identifier to a remote server together with an indication that the user of the device has been verified by the mobile device.
 16. The mobile device of claim 12, wherein: the previously authenticated user is associated with a unique user identifier, and the at least one memory and the computer program code are configured, with the at least one processor, to cause the mobile device to, responsive to determining that the first image and the second image represent the same user, store, in a storage device, an association between the unique user identifier and the second image.
 17. The mobile device of claim 16, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the mobile device to, responsive to determining that the first image and the digital image represent the same user, send the unique user identifier to a remote server whereby to indicate that the user of the mobile device has been verified as the user associated with said identifier.
 18. The mobile device of claim 12, wherein the at least one memory and the computer program code are configured, with the at least one processor, to acquire the first image by causing the camera to capture the first image, the first image corresponding to an image of a portion of the identity document containing a photographic image of the previously authenticated user visible on the identity document.
 19. The mobile device of claim 12, wherein the at least one memory and the computer program code are configured, with the at least one processor, to acquire the first image by causing a chip reader connected to or integral with the mobile device to access the integrated circuit component, whereby to retrieve the data representative of the digital image of the previously authenticated user
 20. A non-transitory computer-readable storage medium, comprising a set of computer-readable instructions stored thereon which, when executed by a processing system of a mobile device, cause the mobile device to perform a method of determining whether a user of a mobile device corresponds to a previously authenticated user, the previously authenticated user having been previously authenticated via an identity document comprising at least one image of the previously authenticated user, wherein the at least one image comprises at least one of a portion of the identity document containing a photographic image of the previously authenticated user visible on the identity document and data representative of a digital image of the previously authenticated user stored on an integrated circuit component of the identity document, the method comprising: acquiring, by the processing system of the mobile device, a first image from the identity document, the first image being one of said photographic image and said digital image; causing, by the processing system of the mobile device, a camera connected to or integral with the mobile device to capture a second image, the second image corresponding to a user of the mobile device; comparing, by the processing system of the mobile device, data indicative of the first image and data indicative of the second image, whereby to determine whether the first image and the second image represent the same user; and responsive to determining that the first image and the second image represent the same user, forming, by the processing system of the mobile device, an association between the previously authenticated user and the mobile device. 